Axially adjustable gas springs are known, for example, from the German laid-open application 27 42 224. In this publication is described a pneumatic spring with supplemental elastic stiffening, i.e., a so-called terminal stiffening, which is attained by embodying the piston of the telescoping spring as an additional, supplementary gas spring. In this way, the device supplies additional springing independently of the relative position of the piston rod with respect to the housing. One of the disadvantages of this known spring is that the supplemental pneumatic spring must be filled with working fluid in a separate step. The basic axially adjustable gas spring without terminal stiffening is known also, for example, from U.S. Pat. No. 3,656,593.
The Belgian Pat. No. 638 166 describes a hydraulic or pneumatic piston cylinder assembly with an axially slidable piston attached to a piston rod that emanates sealingly from the housing. The piston has a continuation which enters a coaxial recess in a sealing manner when the piston is moved into the cylinder. The recess is connected to a source of pressure via a suitable line and the wall of the recess has a lip seal which attaches to the piston continuation. When a pressurized medium is applied to the side of the piston adjacent to the piston rod, the piston enters the housing so that the fluid on the other side of the piston is pushed out of the aforementioned recess. At the moment when the continuation enters the recess however, the lip seal seals it so that the fluid can no longer pass from the volume defined between the piston and the lower part of the housing so that the pressure within the housing rises and increases the force opposing the inward motion of the piston, constituting an auxiliary gas spring opposing final entry of the piston into the housing.
Further known in the art is a simple pneumatic spring described in the British Patent 1 402 849. Described there is a telescoping cylinder/piston assembly with a supplementary guide piston having passages which provide communication between the volumes defined on either side of the piston. A simple pneumatic spring of this type acts substantially like a mechanical helical spring, i.e., its axial extent is not adjustable. However, the pneumatic spring described in the aforementioned publication also has an extension that becomes associated with a cylinder of reduced diameter and located at the closed end of the housing. Accordingly, in the terminal stages of insertion, the piston extension enters the section of reduced diameter and increases the resistance to further insertion as well as providing an increased initial expulsion force.